Through a transmit beamforming (Beam Forming, “BF” for short) or precoding technology, a multiple input multiple output (“MIMO” for short) wireless system may obtain diversity and array gain. A typical system using BF or precoding generally may be denoted as y=HWs+n, wherein y is a received signal vector, H is a channel matrix, W is a precoding matrix, S is a transmitted signal vector, and n is a measurement noise and interference.
An optimal precoding generally needs a transmitter to completely know channel state information (“CSI” for short). As in a common method, a user equipment (“UE” for short) quantizes instantaneous CSI and feeds back the quantized CSI to a base station (evolved Node B, eNB). The CSI information fed back in the existing long term evolution (“LTE” for short) release 8 (“R8” for short) system includes a rank indicator (“RI” for short), a precoding matrix indicator (“PMI” for short) and channel quality indicator (“CQI” for short) etc, wherein the RI and the PMI indicate the number of layers and a precoding matrix used respectively. Generally, a set of precoding matrices is referred to as a codebook or a precoding codebook, and each precoding matrix in the codebook is referred to as a code word.
To reduce system cost, achieve higher system capacity and meet coverage requirement meanwhile, an active antenna system (“AAS” for short) is already widely deployed in practice. AAS is being considered to be introduced into an LTE release 12 (“R12” for short) standard to be started at present, so as to enhance the communication performance of the system. In AAS system, a transceiver is directly connected with antenna elements, and the amplitude and phase of each antenna element may be directly adjusted, so that not only beam direction but also beam shape can be adjusted.
With the development of data services, particularly the appearance of a hotspot region in a heterogeneous network, a network system is required to adapt to a unbalanced network load. For example, better power is provided for the hotspot region, etc. The AAS provides a beneficial manner for solving the load imbalance issuse, so as to focus the power on the hotspot region and reduce the interference on a non-hotspot region at the same time.
However, the present LTE R8 system adopts a single codebook, wherein the 4-antenna codebook is designed on the basis of a Householder transformation, and each element of a precoding matrix in the codebook has the same amplitude, namely satisfies constant modulus property, so that the precoding matrix can only adjust the beam direction but can not adjust the beam shape. LTE release 10 (“R10” for short) system further introduces a double-codebook design for 8 antennas, but the precoding matrix included in double codebooks can also only adjust beam direction but can not adjust beam shape.
Therefore, the above-mentioned codebooks can not enable a communication system to realize a load balance.